3_energy metabolic rate (2014)
DESCRIPTION
HKU science lecture notesTRANSCRIPT
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Energy
A food calorie is a kilocalorie (1000 cal, , ). One kcal is the amount of energy (heat) required to raise
the temperature of 1 kg water by 1o Celsius from 15
to 16o Celsius.
1 kcal = 4.184 kilojoule (kJ, )
Food energy is captured when macronutrients are oxidized.
Oxidation can take place in the chamber of an apparatus or
intracellularly within an organism.
Measuring gross energy (a direct method
to capture energy released)
What chemical reaction takes place
when food is burned?
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[1-9%]
(E intake E feces)/E intake * 100
Energy value of foods (Atwater factors)
Release in metabolism /
futile cycles
[~50%]
[25-40%]
Food energy available for body functions that needs ATP
Alcohol
7 kcal/gm
(6.9 to be exact)
Atwater Factors
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Question: Do you think 1 kcal derived from protein is
same as from fat and glucose?
i.e. Is a calorie a calorie?
Remember, the Atwater factors are AVERAGE values. They are in
error to some degree for basically any food item. The error comes
from the complex structure of food that can change the gross energy as
well as availability.
For example, a food containing high proportion of unavailable dietary
carbohydrate would increase the excretion of protein and fat in the
feces. Thus, the factors overestimate the energy derived from such
foods.
Implication: we cannot determine the exact metabolizable energy of a
given mixed diet.
According to the 1st Law of Thermodynamics, energy can
be neither created nor destroyed, but only transformed.
Hence, food energy that gets inside the body will either be
used (work/heat) or stored. So, body energy balance could
be an estimate of how energy derived from the
macronutrients is utilized.
Energy balance = Output Input
On a long-term basis
Output > Input decrease energy reserve & BW
Output < Input increase energy reserve & BW
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Determinants of energy Expenditure
By the Factorial Method
1) Basal metabolism (basal metabolic rate, BMR)
2) Physical activity
3) Thermic effect of food (specific dynamic action of food,
diet-induced thermogenesis)
4) Growth
pregnancy, lactation, baby, children & teenager
30-50%
Physical activities
10%
Thermic effect
of food 50-65%
Basal metabolism
Components of Energy Expenditure
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Energy expenditure measurement:
1)Direct body heat loss
2) Indirect via O2 consumption or
CO2 production
Indirect calorimetry
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Determinant of energy need - Basal Metabolic Rate (BMR)
The minimum amount of energy
spent to maintain life.
Measured under a quiet,
inactive & non-stress post-
absorptive state.
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Respiratory quotient (RQ) and fuel usage
When our body burns fuel, the amount of
oxygen consumed and carbon dioxide produced
would depend on what is being burned
For carbohydrate, oxygen consumed & carbon
dioxide produced are on equal molar basis
But, for fat, more oxygen is consumed than
carbon dioxide produced - WHY?
RQ = moles carbon dioxide produced / moles
oxygen consumed
RQ for carbohydrate, fat and protein is ~ 1, 0.7
and 0.85, respectively
RQ data will suggest the major fuel being burned
and energy expenditure could also be estimated
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Miscellaneous factors influencing BMR Sex
Developmental stages
age, sexual maturity, pregnancy, lactation
Stress
Diurnal/seasonal cycle (torpor / hibernation)
Thyroid Hormone
Assume one person (A) is tall
and slender and the other (B) is
shorter and wider. However,
they have the same weight.
Will they have similar BMR?
Why?
A
B
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Semi-log plot of specific O2 consumption
against body mass for mammals
the mouse to elephant curve
Surface area as site of
heat loss
Tissue as site of heat
production
The smaller the animal,
the higher the
area : volume ratio
Physical Size
Mass / Size
MR= aWb
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Components of Energy Expenditure Basal Metabolism
W in kg
H in cm
A in year
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Determinant of energy need physical activity
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Effects of Meal Ingestion & Menstrual Cycle
Thermic effect of food
(Diet induced thermogenesis)
Increases in MR due to
mastication
gut movement
digestion & absorption, etc
Note: in each diagram, DIT is the
difference in area under the curves.
Li et al. 1999
Menstrual phase caused
a 16% difference (luteal > follicular)
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Understanding Nutrition, 2011
Question: Is a calorie a calorie?
This could be answered partly by studying the effects of
isocaloric diets on body weight and composition.
To be covered in tutorials